1. Field of the Invention
This invention relates to a microwave frequency discriminator and more particularly to a discriminator comprising a field effect transistor and including a biasing circuit for electronically obtaining impedence matching conditions.
2. Description of the Prior Art
Techniques and devices for the rapid and accurate determination of an unknown signal frequency are of significant interest in modern communication systems, in particular, for electronic counter measure (ECM) systems operating at microwave frequencies. Microwave frequency discriminators capable of converting incoming unknown frequencies into voltages for processing are often used in ECM systems. A microwave frequency discriminator may be defined as a circuit that provides an output voltage which is a predetermined function of and usually proportional to the frequency of an incoming signal. The discriminator voltage output versus frequency response, commonly termed the "discriminator characteristic," is the respone in which the output voltage varies nearly linearly with respect to frequency over a predetermined frequency bandwidth. The bandwidth is generally determined by the slope, linearity, and resolution of the discriminator and is the frequency range over which the discriminator provides an unambiguous voltage output which is related to the input frequency.
A typical prior art broadband microwave discriminator utilizes passive elements such as transmission lines or an arrangement of lumped elements to vary the power level of the input signal applied to a detector diode as a function of frequency. Such a discriminator formed of various passive components, generally, disadvantageously, has a large number of connections between the components. Such interconnections of the components within the disciminator often produce impedance mismatches resulting in undesirable inflection points in the relation between input frequency and output voltage such that a certain voltage can occur at several frequencies. Such distortions in the discriminator characteristic curve limit the broadband resolution of the discriminator minimizing thereby the accuracy of the system.
Most prior art discriminators are preceded with a limiter to provide a constant power input to the discriminator, whereby the output voltage of the discriminator is a function of frequency alone. Limiters may not be required, however, if the incoming signal has a relatively constant magnitude. Since such a prior art passive discriminator network has no gain, the voltage output which it can produce is restricted by the power available from the limiter preceding the discriminator. The overall linearity and usable bandwidth of the discriminator is also restricted by the frequency response and uniformity characteristics of each of the many elements of such a passive discriminator. In addition, the numerous interconnections in the passive discriminator produce multiple wave reflections which cause distortions of the frequency-voltage discriminator characteristic which reduce the accuracy and usable frequency range of the discriminator.